1. Technical Field
The present invention relates to wireless communications, and, more particularly, to managing base-station transmission power in wireless networks.
2. Description of Related Art
a. CDMA Networks Generally
Many people use mobile stations, such as cell phones and personal digital assistants (PDAs), to communicate with cellular wireless networks. These mobile stations and networks typically communicate with each other over a radio-frequency (RF) air interface according to a wireless communication protocol such as Code Division Multiple Access (CDMA), perhaps in conformance with one or more industry specifications such as IS-95 and IS-2000. Wireless networks that operate according to these specifications are also referred to as “1xRTT (1x) networks,” which stands for “Single Carrier Radio Transmission Technology”. These networks (referred to herein as “CDMA networks”) typically provide communication services such as voice, Short Message Service (SMS) messaging, and packet-data communication.
Typical CDMA networks include a plurality of base stations, each of which provide one or more wireless coverage areas, such as cells and sectors. As a general matter, sectors are used herein as examples of wireless coverage areas. When a mobile station is positioned in a sector, it can communicate over the RF air interface with the base station, and in turn over one or more circuit-switched and/or packet-switched signaling and/or transport networks to which the base station provides access. The base station and the mobile station conduct these communications over a frequency known as a carrier. Note that base stations may provide service in a sector on one carrier, or on more than one, and further that a given instance of a given carrier in a given sector may be referred to herein as a “sector/carrier.”
Communication, including voice and packet-data communication, between the mobile station and the base station is separated into forward-link communication (from the base station to the mobile station) and reverse-link communication (from the mobile station to the base station). And each carrier over which this communication takes place is typically actually a pair of distinct frequencies—one for the forward link and the other for the reverse link. This approach is known as frequency division duplex (FDD).
In a typical CDMA network, using a configuration known as radio configuration 3 (RC3), a base station can, on each sector/carrier, transmit forward-link data on a maximum of 64 distinct channels at any given time. Each of these 64 channels corresponds to a unique 64-bit code known as a Walsh code. Of these, typically, 61 channels are available for use as traffic channels (to carry user data), while the other 3 are reserved for administrative channels known as the pilot channel, the paging channel, and the sync channel.
In general, the pilot channel functions to alert mobile stations in a given sector of the presence of a service-providing base station. Typically, the pilot channel conveys a value known as a pseudorandom number (PN) offset, which identifies the sector; in particular, by being offset from CDMA system time by a certain amount of time, the pilot channel conveys the PN offset. Mobile stations generally use the pilot channel to coherently detect and demodulate the signal on a sector/carrier. The paging channel is typically used to transmit overhead messaging, such as incoming-call and message-waiting pages, as well as SMS messages, to mobile stations. The paging channel typically also broadcasts values known as the system identification code (SID) and the network identification code (NID), which, taken together, help identify a given sector. The sync channel generally provides information that enables the base station and mobile stations to be precisely synchronized on CDMA system time.
When a base station instructs a mobile station to use a particular traffic channel for a particular communication session, the base station does so by instructing the mobile station to tune to a particular one of those 64-bit Walsh-coded traffic channels. It is over that assigned traffic channel that the base station will transmit forward-link data to the mobile station during the ensuing communication session. Note that, in addition to including the forward-link channel, the traffic channel also includes a corresponding Walsh-coded reverse-link channel, over which the mobile station transmits data to the base station. These traffic channels may be used for different types of communication, among which are second-generation (2G) voice, 2G data, third-generation (3G) voice, and 3G data.
b. Forward-Link Transmission-Power Management
i. The Power Control Bit and the Ratio Eb/Nt 
In CDMA networks, the transmitting power of a base station on a given sector/carrier is divided among the mobile stations to which the base station is transmitting voice data and/or packet data on traffic channels, as well as among the pilot, paging, and sync channels mentioned above. With respect to mobile stations that engage in 3G voice and 3G data communications, the amount of power that the base station is allocating to the transmission to any one mobile station on that mobile station's respective traffic channel at any given moment is based on a number of factors, two of which are known as the power control bit (PCB) and the ratio Eb/Nt.
The PCB is a bit (0 or 1) that the mobile station sends to the base station on the reverse link at a high frequency, on the order of 800 times per second (i.e. once every 1.25 milliseconds (ms)). The base station repeatedly responsively adjusts the transmission power to the mobile station on the traffic channel. Typically, if the mobile station sends a 0, the base station will decrease the power by a set decrement, such as 1 dB, whereas, if the mobile station sends a 1, the base station will increase the power by a set increment, which may also be 1 dB. Thus, the forward-link transmission power changes every 1.25 ms by plus or minus 1 dB.
A typical CDMA mobile station determines whether to transmit a PCB equal to 0 or 1 by comparing (i) a signal-to-noise ratio that the mobile station repeatedly computes with (ii) a stored threshold value for that signal-to-noise ratio. This ratio is generally known and referred to herein as “Eb/Nt,” while the threshold is referred to herein as the “Eb/Nt threshold.” Eb/Nt essentially compares the strength at which the mobile station is receiving the traffic-channel signal (Eb for “energy per bit”) with the strength at which the mobile station is receiving signals from all sources on the frequency of the sector/carrier (Nt. for “noise”) As stated, then, Eb/Nt is a signal-to-noise ratio for the traffic channel. And the mobile station typically computes Eb/Nt at the same frequency at which it transmits the PCB, which again may be once every 1.25 ms.
Thus, in typical operation, every 1.25 ms, the mobile station compares the most recent computation of Eb/Nt with the Eb/Nt threshold. If Eb/Nt exceeds the threshold, then the mobile station is receiving a strong enough signal on the traffic channel, and thus it transmits a PCB of 0, which causes the base station to reduce the forward-link power on that traffic channel. If, on the other hand, the computed Eb/Nt is less than the threshold, the mobile station is not receiving a strong enough signal, and thus it transmits a PCB of 1, which causes the base station to increase the forward-link power. Thus, the forward-link power on the traffic channel typically stabilizes to a point that achieves an Eb/Nt value (as measured at the mobile station) that is near the Eb/Nt threshold. And this threshold can be changed during operation.
ii. Forward-Link Frame Error Rate (FFER)
In CDMA networks, data is transmitted from the base station to the mobile station (and vice versa) in data units that are known as frames, which typically last 20 ms. Some frames received by mobile stations contain errors as a result of imperfect transfer from the base station, while some do not. The forward-link frame error rate (FFER) is a ratio, also computed by the mobile station, of the number of error-containing frames that the mobile station receives to the total number of frames that the mobile station receives, over a given time period. Note that the FFER often also takes into account frames that are not received at all by the mobile station. And, other things being more or less equal, the more power that the base station allocates to a given mobile station, the lower the mobile station's FFER will be.
More particularly, at approximately the same frequency at which the mobile station is receiving forward-link frames (i.e. once every 20 ms), the mobile station computes its FFER over some previous number of frames, which may be 20, 100, 200, or some other number. Thus, the mobile station essentially computes a FFER for some rolling window of previous frames. And each time the mobile station computes its FFER, the mobile station compares that computed value with a threshold, often referred to as the “FFER target,” which may be around 2%.
If the FFER exceeds the FFER target, the mobile station is experiencing too many error-containing and/or missed frames, and thus the mobile station will responsively increase its Eb/Nt threshold. In the short term, this will result in the mobile station's computed Eb/Nt falling below that increased threshold, which in turn will result in the mobile station repeatedly sending PCBs equal to 1 to the base station. This, in turn, will result in the base station increasing the forward-link transmission power to the mobile station on the traffic channel. The forward-link power will then typically stabilize at a level that will result in the mobile station computing an Eb/Nt that is very close to its new Eb/Nt threshold, and perhaps result in an acceptable FFER.
If, on the other hand, the FFER falls below the FFER target, the mobile station may be drawing excessive power from the base station on the forward-link—in essence, the mobile station's signal may be considered too strong, perhaps at the expense of other mobile stations on the sector/carrier. If that situation holds for a specified period of time, the mobile station may decrease its Eb/Nt threshold, which will result in the mobile station's computed Eb/Nt repeatedly exceeding that decreased threshold. This, in turn, will result in the mobile station repeatedly sending PCBs equal to 0 to the base station, which will result in the base station decreasing the forward-link transmission power to the mobile station on the traffic channel. Again, the forward-link power will then typically stabilize at a level that will result in the mobile station computing and Eb/Nt that is very close to its new Eb/Nt threshold.
Thus, the combination of (i) the mobile station's FFER target and (ii) the mobile station's repeated FFER calculation and comparison with the FFER target causes the mobile station to iteratively adjust its Eb/Nt threshold. In turn, the mobile station's even-more-frequent calculation of Eb/Nt and comparison with its current Eb/Nt threshold causes the mobile station to iteratively send power control bits of 0 (for less power) or 1 (for more power) to the base station, which then causes the base station to adjust the forward-link power on the traffic channel. This entire back-and-forth calibration process is conducted in an attempt to keep the mobile station's FFER at or below what is deemed to be an acceptable threshold, which again may be around 2%.
Note that different situations may present themselves on a given sector/carrier at different times. For one, the number of mobile stations using traffic channels can vary between just a few, such as 10, to a larger number, such as 30, and perhaps approach the upper bound of 61 (assuming RC3). And, as stated, the power that the base station allocates for transmission to these mobile stations can vary. In particular, variables such as terrain, weather, buildings, other mobile stations, other interference, and distance from the base station can affect the FFER that each mobile station experiences, and thus the amount of power the base station allocates for each mobile station. Since base stations have a finite amount of power that they can allocate to the mobile stations on a given sector/carrier, increasing the transmission power to some or all of those mobile stations generally results in the base station being able to serve fewer mobile stations on that sector/carrier. That is, it reduces capacity on the sector/carrier.
iii. The Logarithmic Ratio Ec/Ior 
As explained, in CDMA networks, a given base station has a finite amount of power for transmitting on each sector/carrier on which it provides service. The base station divides this power among any active traffic channels (over which it is transmitting voice and/or packet data to mobile stations), as well as among the pilot, paging, and sync channels. Periodically, for a given sector/carrier, the base station calculates a ratio of (a) the power it is allocating for transmitting the pilot channel (the “pilot-channel power level”) with (b) the power it is allocating for transmitting all (i.e. pilot, paging, sync, and traffic) channels (the “all-channel power level”).
This ratio is a base-10 logarithmic one, and is known as “Ec/Ior.” The pilot-channel power level is referred to as “Ec”—“energy per chip.” The all-channel power level is referred to as “Ior”. Ec and Ior can each be expressed in Watts (W), milliwatts (mW), or any other suitable units of measure. Note that Ec and Ior are often expressed as base-10 logarithmic ratios themselves, with respect to a reference power level of 1 mW. In that case, Ec and Ior would each typically be expressed using the unit “dBm,” where “dB” indicates decibels and “m” indicates the reference power level. So, Ec can be expressed as the base-10 logarithmic ratio of the pilot-channel power level (in mW) and 1 mW. And Ior can be expressed as the base-10 logarithmic ratio of the all-channel power level (in mW) and 1 mW.
Ec/Ior is typically expressed as the base-10 logarithmic ratio of the pilot-channel power level and the all-channel power level, each of which may be measured in Watts. As such, the typical unit of measure for Ec/Ior is the decibel (dB). As an example, if a base station were allocating about 2 W (2000 mW) for the pilot channel, Ec would be about 33 dBm, calculated as 10*log((2000 mW)/(1 mW)). And if the base station were allocating a total of about 10 W (10,000 mW) for the pilot, paging, sync, and active traffic channels, Ior would be about 40 dBm, calculated as 10*log((10000 mW)/(1 mW)). In this example, Ec/Ior would be about −7 dB, calculated as 10*log((2 W)/(10 W)). Note that Ec/Ior will always be negative, as long as at least some power is allocated for any one or any combination of the paging, sync, and traffic channels.
As another example, a typical base station may have 16 W of power that it can potentially use for transmitting all channels on a sector/carrier, and may allocate 15% (2.4 W) of that for the pilot channel, 10% (1.6 W) for the paging channel, and 5% (0.8 W) for the sync channel. When that base station is not serving any mobile stations on active traffic channels on the sector/carrier, i.e. when the sector/carrier is “unloaded,” Ec/Ior would be approximately −3 dB, calculated as 10*log((2.4 W)/(4.8 W)), which, then, would be about as high as Ec/Ior gets. Thus, for reference, anything close to −3 dB may be considered relatively high for Ec/Ior.
And when that same base station is at or near capacity (“fully loaded”), the 15% of its potential sector/carrier power that it is allocating for the pilot channel would shrink from being half of its power output on the sector/carrier (in the unloaded scenario) to, not surprisingly, being about 15% of its power output. This would yield an Ec/Ior of approximately −8 dB, calculated as 10*log((2.4 W)/(16 W)), which, then would be about as low as Ec/Ior gets. Thus, for reference, anything close to −8 dB may be considered relatively low for Ec/Ior. In fact, a typical base station may stop accepting new mobile stations on a sector/carrier once Ec/Ior degrades to about −8 dB. Thus, Ec/Ior can impact sector/carrier capacity as well.
When Ec/Ior is relatively high, this could mean a number of things. For example, there could be only a few mobile stations on the sector/carrier (i.e. the sector/carrier is lightly loaded), which would generally lead to a higher ratio of pilot-channel power allocation to total power allocation (with relatively few traffic channels to which to allocate power). Instead or in addition, it could mean that RF conditions are favorable, such that no (or relatively few) mobile stations are experiencing a poor FFER. In that situation, there would be no (or relatively few) mobile stations inducing the base station to increase power on the traffic channels. This would tend to keep the value of Ec/Ior relatively high. And other possibilities exist as well.
When Ec/Ior is relatively low, this also could mean a number of things. For example, there could be a relatively high number of mobile stations on the sector/carrier (i.e. the sector/carrier is heavily loaded), and thus a high number of active traffic channels contributing to a high value of Ior, and thus a low value of Ec/Ior. Instead or in addition, it could mean that RF conditions are poor (e.g., due to terrain, weather, interference, etc.); in that case, mobile stations would likely experience poor FFER, and induce the base station to increase power on the traffic channels, which would contribute to a higher Ior and thus a lower Ec/Ior. And other possibilities exist as well.